In it he reportedly said: "I was dismayed by the BBC Radio 4 Today interview conducted by Evan Davis in which he failed to allow Mr Duncan Smith to finish his answers, asked loaded rhetorical questions – ‘why are you punishing people?’ – and gave a histrionic aside to listeners: ‘This is so frustrating!’.”

He claimed Davis was guilty of a "bizarre display of petulance" during the interview. And in another Caseby letter reported by the Mirror, he is quoted accusing the BBC of "lack of balance and fairness" when covering welfare reforms.

Caseby has previously blasted The Guardian for a series of inaccuracies in its coverage of his department.

Fran Unsworth, Deputy Director BBC News and Current Affairs, said: “It was certainly evident to me that the interview was robust and that Evan Davis interrupted several times. But a challenging interview does not in itself demonstrate bias; it is about holding people in power to account whichever party they come from. Nor do I think the questions were rhetorical, rather than testing.”

James Harding, Director BBC News and Current Affairs, said: “In the run up to the General Election, I think it extremely important not just that the BBC reports accurately but that politicians and civil servants respect the BBC’s right and responsibility to deliver independent and impartial news.”

UPDATE:

Press Gazette understands that Caseby also complained about a number of mistakes in a Today programme investigation into the spare room subsidy which was broadcast on 28 March.

In a letter, Unsworth is understood to have said: "I am not generally in favour of presenters commenting on the progress of an interview as listeners should be left free to make up their own minds about it…"

She is also understood to have said: "We are sorry about these mistakes and I apologise for them."